For many years considerable effort has been directed at increasing the number of print wires in an assembly (package) to increase the resolution and throughput of printing and, at the same time, at configuring the printing elements so as to achieve higher velocity and frequency response.
Most of the commercially available wire matrix print head assemblies have been of a generally circular design in which the force generating elements such as electromagnets (solenoids) have been mounted in radial orientation about a central axis. Each electromagnet drive element generally has a single energizing coil mounted about one of the yoke poles for attracting a radially oriented armature in a pivotal motion about the yoke's outer (or inner) end. As the armature is pivoted, its inner end engages and drives a respective print wire from a retraced position to a forward, printing position.
The drawback of the usual single-coil configuration is that while the space in the peripheral area of the circular package is packed tightly, in order to keep the size of the package at the minimum (for a given number of actuators in a package), a vast area closer to the axis of the package remains virtually unused.
Attempts to improve the use of space in a package for generating useful energy have been made in some designs, in particular in wire print heads described in U.S. Pat. No. 4,382,701 of May 10, 1983, granted to Keith B. Davenport et al., and U.S. Pat. No. 4,218,148 of Aug. 19, 1980, granted to Arthur L. Matachka. A pair of radially spaced coils of a generally identical cross-sectional dimensions in each of the actuators is described in both of the patents. The coils are mounted next to each other on two adjacent limbs (poles) of the respective yokes. Though an additional coil in an actuator may contribute to some improvement of the print heads performance, the solution is far from ideal. For example, a two coil electromagnet, having a U-shaped core and a common to both poles armature, establishes a closed magnetic path requiring that the two coils have an opposite polarity. The opposite polarity means that the magnetic flux generated by the two coils in the space between them is mutually cancelled, resulting in a significant part of the supplied power being lost. Additionally, a dual-coil arrangement with the coils of a generally identical cross-sectional dimensions does not provide for a maximal usage of space in a package because the profile of the inner coils determines the maximum number of actuators that can fit in a package of a given diametrical size, or the size of a package with a given number of actuators. The outer coils inevitably are spaced apart much farther than they would be in a single coil arrangement, which means that a significant part of the space available in a package is lost.
It is the principal object of the present invention to provide a wire matrix print head assembly that dramatically increases the number of print wires for a high resolution and high throughput printing while minimizing the size of the package and increasing the impact force and the frequency response of the print wires.
Additionally, many wire matrix print head assemblies utilize O-rings engaging the ends of the armatures to facilitate both the pivotal motion of the armatures and shock absorption in their return stroke. Such O-rings, for example, are illustrated in U.S. Pat. No. 4,051,941 granted to Donald G. Hebert on Oct. 4, 1977. It has been found, however, that the O-rings transmit vibration from one armature to another circumventially, which impairs performance of the print heads. Attempts to partially overcome this problem by having individual shock absorbers for each armature, separately mounted on a common element of the structure, such as the armature retainer, are represented by U.S. Pat. No. 4,214,836 of July 29, 1980 granted to Cheng H. Wang and U.S. Pat. No. 4,382,701 to Keith B. Davenport et al., referred to above. The drawback of the individual, separately mounted shock absorber designs is high cost of manufacture and assembly of a number (up to 24, and more) of parts vs. a single part like an O-ring.
It is another object of the present invention to provide a highly efficient wire matrix print head assembly having low cost, easily manufactured and easily mounted elements facilitating the pivotal motion of the armatures and the shock absorption in their return stroke, without transmitting vibration from one armature to another.
These and other objects and advantages of this invention will become apparent upon reading the following detailed description of a preferred embodiment.